That extreme heat you're feeling on a disgustingly sweaty Summer's day? It might be all in your head. Well, mostly.

Wired points to a study by the European Journal of Applied Physiology who tested cyclist performance at various levels of heat shoving thermometers up their butts and letting them bike for awhile (fun!):

The control trial was conducted in a room kept at 71.2 degrees Fahrenheit. A second "hot" trial was held in a room at 88.5 degrees. The final one was a "deception" trial, in which the temperature was displayed as 78.8 degrees but it was actually 88.8 degrees, the hottest of the three. The trials were administered in a randomized way, and all seven subjects performed all three. (Rectal thermometers used to measure each cyclist's core body temperature were also displayed as being slightly lower than what they were actually were.)

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The results? The cyclists performed better at 71.2 degrees than they did at 88.5 degrees, but when they deceived into thinking it was 78.8 degrees when it was really 88.8, they actually performed best of all (by a very small margin). Basically, so long as the cyclists thought it wasn't really hot they performed just as well.

Next time you're out on a hot day and you don't know how hot it really is, don't check. You might be better off.